Association Between The Posterior Part Of The Circle Of Willis And Vertebral Artery Hypoplasia

Background It is not clear whether the configuration of the posterior part of the circle of Willis (CW) depends on the proximal part of the vertebrobasilar system. Our aim is to evaluate the posterior part of CW in association with different size of vertebral arteries (VA) in subjects free from stroke and TIA. Materials and methods The present study was based on a sample of 923 subjects free from stroke and TIA who were examined from 2013 through 2018. All the participants underwent MRA examination. The duplex ultrasonographic examination of the extracranial arteries (vertebral and carotid) was performed. VA was defined as hypoplastic (VAH) when VA diameter in the entire course was less than 2.5 mm. We classified the posterior communicating arteries (PCoA) as presence PCoA, absence/hypoplastic PCoA and fetal CW (FCW). The comparison of the posterior part of CW was made in subjects with normal VA and VAH of a different degree (communicating with basilar artery (VAH-BA) and not communicating with the basilar artery and terminating in PICA, neck or aplasia (VAH-PICA)). Results FCW was found in 15.9% of subjects, bilaterally – in 2.3 %. The coexisting VAH was more common in subjects with FCW rather than in those with adult CW (respectively, 28.6% and 13.4%, p<0.001). Aplasia of A1 of the anterior cerebral artery, i.e. blood flow redistribution in the anterior part of anterior circulation in the majority of cases (in 6 of 7 cases) was found ipsilaterally to FCW. FCW was recorded in 50% of the subjects with VA - PICA in comparison with 13.5% of those with normal VA and 22.8% with VAH - BA, p<0.005. On the contrary, absence/hypoplasia of both PCoA was mostly found in the group with normal VA in comparison with VAH-BA and VAH-PICA (accordingly, 50.7%, 38.6% and 12.5%, p<0.01). Conclusion Individuals with VAH have a different pattern of the posterior part of CW in comparison with those with normal VA. With the increasing degree of VAH, the proportion of FCW increases, while the proportion of absence/hypoplastic of both PCoA decreases.

anatomy in which PCA arises directly from the terminal ICA, with or without an intact P1 63 segment connecting PCA to the basilar artery. In this variant, the larger brain area is 64 dependent on ICA and could be more prone to develop large ischemic strokes in cases of 65 carotid artery stenosis or occlusion. As described by many authors [1], in these cases the 66 collateral circulation between the anterior and posterior circulation through secondary 67 collaterals, i.e. leptomeningeal vessels cannot develop since both, the middle cerebral artery 68 and PCA are connected to the same internal carotid system.

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The present study was based on a sample of 923 subjects without cerebrovascular disease 85 (TIA or stroke) history before and at the time of the study enrollment. All of them were 86 examined by magnetic resonance imaging (MRI) and magnetic resonance angiography   Imaging studies. 94 All the participants underwent MRA examination using 1.5 Tesla MRI (GE Optima MR450w 95 1.5T MRI System) for the brain and CW evaluation.

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The following sequences were obtained: 3D T1 weighted, T2 FLAIR, T2 weighted, diffusion  The duplex ultrasonographic examination of extracranial arteries (vertebral and carotid) was 101 performed by using the 7.5 MHz linear array transducer of Aloka Prosound F 75 ultrasound 102 system. The diameter of VA in our previous study was measured similarly [4]. 103 Image analysis. 104 MRA were reviewed by two independent neuroradiologists. If they had disagreements 105 regarding the configuration of the circle of Willis, they discussed it until a consensus was 106 reached.

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The classification of CW and VA was carried out as follows:       The Chi square independence (χ2) test was applied in carrying out the comparison between 146 the categorical variables, while Fisher's exact test was used in the case of a small sample size.

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Continuous variables meeting the assumptions of normality were analysed using t-tests for 148 independent groups. The chosen significance level was α=0.05.

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Ethics. This study was approved by the Ethics Committee for the Vilnius region (No.   Table   155 1. FCW was found in 15.9 % of subjects free of stroke and TIA. Side-related differences in   proportion ranges from 1.9% to 25% [10]. Therefore, the population under our investigation 207 was a typical population.

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FCW was more frequently observed in subjects with VAH, i.e. with an insufficiently 209 developed proximal part of the vertebrobasilar system, compared to those with normal VA 210 diameter. Among individuals with a very small VA terminating in PICA/neck/aplasia, 211 compared to those subjects whose VA is wider and forms the basilar artery, the proportion 212 of FCW was larger. Moreover, FCW was more common in ipsilateral to VAH side rather 213 than contralateral. and as a consequence the demand to compensate the possible inadequate blood supply to the 220 brain is under discussion. Although, many authors estimate VAH as an independent predictor 221 of stroke or TIA [10]. The hypothesis that VAH can lead to the posterior circulation 222 insufficiency is also supported by our results that with the decreasing VA diameter, the risk 223 of stroke/TIA increases [4]. Moreover, VAH can lead to a relative regional hypoperfusion in 224 the PICA territory [11]. As described in a study [1], during the embryological development 225 the anterior circulation supplies the occipital region, the brain stem and the cerebellum via  Future investigations are needed in order to assess whether in cases of VAH the configuration 249 of the posterior part of CW can prevent or increase the stroke/TIA risk. The study [1] 250 revealed that the coexistence of the basilar artery hypoplasia, VAH and the fetal CW were 14 14 251 more common in stroke patients. However, in the above mentioned study, the role of FCW is 252 not clear. Is FCW an independent stroke predictor, or is FCW not able to compensate the 253 reduced blood flow in cases of coexistence of small proximal and distal parts of the 254 vertebrobasilar system? Future investigations are needed on the associations between a small 255 vertebrobasilar system, CW configuration and neurological symptoms such as vertigo.

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Patients with stroke/TIA were excluded from our study, however, suggestions can be made 257 for further studies to compare how CW differs in vertigo patients and healthy subjects.

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In cases of FCW, the territory supplied with blood by the carotid artery increases up to three